tools/fs-common.c: workaround error at end of file

OpenBSD requests a chunk of data at the end of the file. Currently
sqsh_file_reader_advance() returns an error in this case. This
commit works around this behavior by returning early when
zero size is requested.

tools/fs-common.c

@@ -145,6 +145,15 @@ fs_common_read(
 		size = file_size - offset;
 	}
 
+
+	// This works around a bug in sqsh_file_reader_advance:
+	// Reading a 0 size at the end of the file will return
+	// SQSH_ERROR_OUT_OF_BOUNDS. This should succeed instead,
+	// providing an empty buffer.
+	if (size == 0) {
+		return 0;
+	}
+
 	rv = sqsh_file_reader_advance(*reader, offset, size);
 	if (rv < 0) {
 		goto out;